The present invention relates to apparatus for the manufacture of glass in sheet or ribbon form, the apparatus being of the type composed of a glass melting furnace from the end of which molten glass is fed to a narrower exit channel which supplies the glass to sheet or ribbon forming apparatus. The invention also relates to a method of manufacturing glass in sheet or ribbon form using such apparatus and to glass sheets or ribbons which have been manufactured by that method.
While such systems for feeding molten glass from a furnace to sheet or ribbon forming apparatus may be used in the manufacture of drawn glass, they are a particular feature of many float glass manufacturing processes.
As a result of convection phenomena which are apparent in the molten glass in the furnace, the current of glass which flows forward into the channel towards the sheet or ribbon forming apparatus is often not sufficiently uniform, and this lack of uniformity can cause unacceptable optical defects in the sheet of glass which is formed.
These convection phenomena arise as a result of temperature differences in the bath of molten glass in the furnace and manifest themselves as different currents in the glass. Glass in the upper part of the bath advances as a forward current towards the exit channel, and glass in the lower part of the bath flows as a return current back towards the furnace heat source. The amount of glass flowing in the forward current is in general greater than the quantity of glass which is removed from the furnace and passed to the forming apparatus, and the excess glass forms the return current.
This excess glass is in particular removed from the forward current following contact of the glass with the cool side walls of the furnace. The glass which is cooled in this way becomes denser and flows towards the sole of the furnace to feed the return current. Also, if the inlet end of the exit channel is narrower than the main body of the furnace, for example if the entrance has a width of one half or less the width of the furnace forehearth, as is disclosed, for example, in U.S. Pat. No. 3,597,178, pockets of cooler glass can be collected in the "shoulders" of the end of the furnace to either side of the channel. Glass in these shoulder corners is likewise cooled by contact with the walls, and descends to join the return current.
At the borders between the forward current and the return current, turbulent convection motion is set up, with the result that, in particular, cool currents of glass can be entrained by the forward current and passed to the sheet or ribbon forming apparatus, and this in turn has a strongly adverse effect on the optical quality of the resulting sheet or ribbon.